Light-emitting diodes (LEDs) are semiconductor devices that convert electrical energy directly into visible light of various colours. With the advent of high-flux LEDs, luminaires are progressively being moved from the traditional incandescent or fluorescent lamps to LEDs for increased reliability, higher luminous efficacy and lower maintenance costs. LED-based luminaires are increasingly becoming the architecture of choice in a variety of mainstream commercial applications such as accent lights, wall washing, signage, advertising, decorative and display lighting, facade lighting, and custom lighting, for example.
LEDs are also being used as energy-efficient and long-lived replacements for cold cathode fluorescent lamps (CCFLs) currently employed for backlighting of liquid crystal display (LCD) panels for televisions and computer monitors. Unlike CCFLs which have relatively broadband spectral power distributions, the narrow spectral bandwidths of red, green and blue LEDs can be suited for the corresponding colour filters of LCD panels.
While colour LEDs, for example red, green and blue LEDs, can be used to generate white light for use in LED-based luminaries and LCD panel backlighting, the white light's chromaticity is dependent on the combination of intensities and dominant wavelengths of the LEDs which are combined to produce white light. These optical parameters can vary even when the LED drive current is constant, due to such factors as heat sink thermal constants, changes in ambient temperature, and LED device aging.
One solution to this problem is to employ optical feedback to continuously measure the white light intensity and chromaticity and adjust the drive currents of the LEDs of various colours such that the intensity and chromaticity of the white light remains substantially constant. This solution requires a reliable and relatively inexpensive means of measuring both intensity and chromaticity.
One approach for measuring intensity and chromaticity relies on tristimulus colour sensors such as those manufactured by Hamamatsu™ and TAOS™. These tristimulus colour sensors typically comprise a colourimeter comprising three sensors (typically silicon photodiodes) whose spectral responsivities are modified by dyed colour filters to approximate the Commission Internationale de l'Eclairage™ (CIE) red ( x), green ( y), and blue ( z) colour matching functions of the human visual system, and wherein the combination of filters with photodetectors represent a tristimulus colour sensor. The colourimeter thereby determines the intensity and chromaticity of incident white light by measuring the sensor output with a suitable electrical device, for example a current meter. While it can be difficult and expensive to manufacture suitable filter-photodetector combinations to approximate the colour matching functions of the human visual system, tristimulus colour sensors may be used to directly measure white light intensity and chromaticity. For example, the y colour matching function is equivalent to the CIE V(λ) spectral luminous efficiency function for photopic vision, and therefore represents luminous intensity.
In practice, however, the spectral responsivities of commercial tristimulus colour sensors such as those manufactured by Hamamatsu™ and TAOS™ can only roughly approximate the CIE colour matching functions. If the dominant wavelengths and spectral power distributions of the LEDs of various colours (such as red, green and blue) are fixed and roughly correspond to the peak wavelength responsivities of the tristimulus colour sensor, the three outputs of a tristimulus colour sensor can be used to measure the intensities of the various colours generated by the LEDs. On the basis of this information, the intensity and chromaticity of the resultant white light can be approximately calculated.
There are however three complicating factors. First, both the spectral power distributions of the colour LEDs and the spectral responsivities of the filter-photodetector combinations overlap, so there can be optical crosstalk between the three output channels of the tristimulus colour sensor. For example, the green channel of the tristimulus colour sensor will respond to radiant flux emitted by a blue or red LED.
Second, white light generated by red, green, blue, and amber LEDs is known to have better colour rendering properties than white light generated by red, green, and blue LEDs. The contribution of the amber light flux to the white light results in a composite spectral power distribution that more closely approximates that of a blackbody light source, which by definition has a CIE colour rendering index of 100. However, the red and green channels of the tristimulus colour sensor generally exhibit significant responses to the amber LEDs. The intensity of the amber LEDs therefore cannot be determined unless the intensities of light generated by the red and green LEDs and their contributions to the red and green channel outputs are known.
Third, even if the spectral power distributions of the colour LEDs and the spectral responsivities of the filter-photodetector combinations of the tristimulus colour sensor do not overlap, any change in the dominant wavelengths of the light produced by the LEDs can result in changes in the tristimulus sensor output. Even if the light-emitting sources are wavelength-tunable monochromatic lasers, the responsivities of the filter-photodetector combinations typically are not constant with respect to wavelength, and the tristimulus sensor output will therefore vary as each laser's wavelength is changed. This problem can be partially alleviated by using thin-film interference filters that have essentially constant bandpass characteristics within a specified range of wavelengths. When used with monochromatic LEDs, these filters can eliminate to some extent the optical crosstalk between channels of the tristimulus sensor. However, LEDs used in lighting applications typically have spectral full width half maximum values of between 15 and 35 nm, so optical crosstalk will typically occur unless the spectral power distribution of a colour LED is completely within the wavelength range of its corresponding colour filter. If the LEDs' spectral power distributions themselves overlap, for example as occurs with red and amber LEDs, optical crosstalk will be unavoidable with tristimulus colour sensors.
Another proposed approach is to use a spectroradiometer, wherein incident white light illuminates a slit and a diffractive element disperses the polychromatic light onto a linear sensor array whose photosensitive elements are sequentially measured by a measuring instrument such as a current meter. To be useful, the spectral resolution of the spectroradiometer must be better than the smallest acceptable change in dominant wavelength in order to avoid perceptible colour shifts in the white light. However, most spectroradiometer designs require precision optics and a considerable volume of space that is incompatible with microelectronic subsystems. Moreover, most of the existing spectroradiometer designs are typically difficult to fabricate, especially those based on micromachined moving parts.
Regardless of the spectroradiometer design, the sensor output typically comprises many different photodetector readings for each spectral wavelength range of 10 nm or less that are assembled into a relative spectral power distribution and then analyzed to determine the relative intensity and dominant wavelength of each LED. The processing power needed to perform this analysis generally requires a fast microprocessor, without which, the processing time may prevent the spectroradiometer from being used for real-time applications where the input signals change over a period of milliseconds.
What is clearly needed is a device with the simplicity and potential ease of manufacture of colourimetric sensors, but which does not suffer from the problem of varying output with changes in dominant wavelength. The spectroradiometer approach fails in that such devices are generally complex and expensive to manufacture, and they generate an overabundance of data that must be analyzed to obtain a few significant values, for example LED intensity and dominant wavelength.
U.S. Pat. No. 4,238,760 to Carr teaches a plurality of photodiodes that are constructed vertically on a common semiconductor substrate, whereby each photodetector exhibits spectral responsivity to different regions of the electromagnetic spectrum. The photodiode design disclosed by Carr has also been extended to implement tristrimulus photodiode arrays, such as those disclosed by Turner et al. in U.S. Pat. No. 6,864,557. A disadvantage of the photodiode design disclosed by Carr is that it can be difficult to obtain predetermined and desirable spectral responsivities solely through the use of semiconductor manufacturing techniques. For example, the photodiode design disclosed by Carr exhibits broad spectral responsivities for the blue and red photodiodes. As a result, the spectral resolution of Carr's photodiodes may be poor, particularly in the presence of electrical noise.
Therefore there is a need for a new multicolour chromaticity sensor that is relatively simple, while providing the desired level of detection.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.